Gas turbine engine exhaust ducts include a liner disposed between an exhaust gas path and an engine casing. Cooling air flows between the liner and casing and is then discharged over nozzle seals and flaps located at the rear end of the exhaust duct to minimize damage that might be caused by high temperatures of the exhaust gas. In order to improve engine operation and performance, duct cooling air is carefully rationed. Since cooling air extracted from the engine may not be utilized to produce thrust, this cooling air may be a penalty to overall engine performance.
To control the quantity of cooling air flow while the engine is subjected to core pressure gradients at various operating conditions, a rotational flow balance system includes a member that is rotated to meter the flow cooling air flow through the duct. The member opens and closes a plurality of flow paths for the cooling air.